1. Field of the Invention
The present invention relates to an image recording apparatus comprising at least one recording unit equipped with a plurality of nozzle arrays featured with a plurality of nozzles for jetting a single color ink to a recording medium and recording an image by means of a relative move between the recording medium and recording unit, and in more specific, to a correction technology for an image recording by an image recording apparatus comprising a plurality of short recording head having a nozzle array or a long recording unit (i.e., a line head) by arraying a plurality of nozzle arrays.
2. Description of the Related Art
An inkjet recording method is known as a recording method by using an image recording apparatus recording an image on a recording medium by jetting ink from each nozzle array of a nozzle array featured with a plurality of nozzles. Such an inkjet recording method applying pressure to ink in an ink fluid chamber in accordance with record information (i.e., record data) by using an ink fluid chamber and a recording head featured with nozzles connected to the ink fluid chamber, thereby jetting an ink droplet from the nozzle and recording an image by fixing the ink droplet onto a recording medium such as paper and film.
On the recording head, there is a trend of multiplication of recording elements (i.e., jet nozzles) and that of elongation of a nozzle array in order to satisfy a demand for speeding up the image recording.
A configuration arraying (i.e., featuring) a plurality of jet nozzles across the entirety of one side of a recording medium, that is, using a line head, is known as an image recording apparatus satisfying such a demand for elongating a nozzle array.
A full line type image recording apparatus using the line head is capable of recording an image on the entire surface of a recording medium just by relatively moving the recording medium and the line head in the direction perpendicular to the array direction of nozzles of the aforementioned line head. Therefore, the full line type image recording apparatus using the line head is capable of recording an image rapidly by a simple operation requiring no movement of a carriage comprising a recording head or an intermittent conveying of a recording medium such as a serial type image recording apparatus.
A line head, however, has a shortfall such as a higher cost, lower production yield and lower reliability than a short recording head.
What is known as an inkjet image recording apparatus solving these problems is a configuration using a line head arraying a plurality of nozzle arrays featured with a plurality of nozzles across a length of no less than the width of a recording medium or with a short recording head comprising the aforementioned nozzle array in the arraying direction of the nozzle array. In such an inkjet image recording apparatus, the advantages such as cost, production yield and reliability by using a short recording head are provided and furthermore the advantage of a high speed image recording by using a line head is also provided.
Also, such an inkjet image recording apparatus performs a relative move (i.e., a move of a recording medium) between the recording medium and the nozzle array of the line head in an approximate fixed velocity usually by using a motor and drives a plurality of nozzles comprising a nozzle array constituting the line head respectively on the basis of the recorded data, thereby performing a dot recording (i.e., an image recording) on a recording medium.
Incidentally, an encoder is usually used for relative position detection and a move velocity control between the recording medium and line head.
Here, if a recording medium on which an image is recorded by the inkjet image recording apparatus is a continuous sheet such as a roll paper, a running start time extending in a large period of time before the conveying velocity of the roll paper reaches at a prescribed fixed velocity.
In order to make the conveying velocity of the roll paper a prescribed fixed velocity in a short period time (i.e., a small running start time), a large conveying drive power is required, facing a large scale of equipment and a high cost thereof.
Meanwhile, in a method of setting a large running start time before the conveying velocity of the roll paper reaches at a prescribed fixed velocity for avoiding a large scale of equipment and a high cost thereof, a high running cost is ushered in due to an increase of an area at the head part of the roll paper, et cetera, where an image recording is not possible, and also a problem of a reduced throughput of the image recording equivalent to an extended running start time.
And the above described serial type image recording apparatus is configured to make the carriage stop once at both ends of the carriage movement range and move in the reverse direction, and therefore set the both end zones of the movement range as acceleration/deceleration area and make the area between the both end areas as a fixed velocity area. Therefore, the serial type image recording apparatus treats only the section of the fixed velocity area as the recording area and therefore is faced with the problem that an image recording time is extended in the amount of time required for moving to the acceleration/deceleration area at both ends of the fixed velocity area and also that the equipment becomes large as much as securing a space for the acceleration/deceleration area.
Consequently, what is desired of the inkjet image recording apparatus of which a recording medium is a continuous sheet such as a roll paper is a capability of recording an image not only in the fixed velocity area, but also in the acceleration area or deceleration area, of the relative move between the recording head and recording medium.
In the meantime, an encoder comprised by an inkjet image recording apparatus comprises the function of generating a timing (i.e., a recording timing) at the time of having the nozzle array of the recording head jetting an ink droplet onto a recording medium.
Making a higher resolution of an encoder ushers in a cost increase of equipment, and therefore a resolution of an encoder output from the encoder used for generating a recording timing is usually lower than a dot density to be recorded. The resolution is expressed by the unit of dpi (a dot per inch) and, as an example, a recording resolution is in the neighborhood of 600 dpi to 1200 dpi for an encoder output of 150 dpi. Thus, because the recording resolution is higher than the resolution of the encoder output, it needs to be multiplied by some method in order to generate a recording timing based on the encoder output.
As one example of a conventional technique of the kind, a reference patent document 1 (i.e., a Laid-Open Japanese Patent Application Publication No. 2004-34650) has disclosed a serial type inkjet image recording apparatus which is configured to obtain an amount of correcting a position caused by a velocity of a carriage while considering an inertial move due to a movement of the carriage at the time of a jetted ink droplet flying toward the recording medium.
FIG. 1 illustrates the method for calculating a position correction amount disclosed in the patent document 1.
Referring to FIG. 1, shown is how the positions of an ink droplet 3 is shifted by a relative move between a nozzle array 1 and a recording medium 2 when flying from the nozzle array 1 toward the recording medium 2.
While the nozzle array 1 moves at the move velocity Vs in the relative move direction (of the x direction), the ink droplet 3 is jetted to the direction (of the z direction) of the recording medium at the velocity Vd. The ink droplet 3 is flown in the direction of a resultant vector of these directions. Therefore, defining the distance between the nozzle array 1 and recording medium as G, the shift amount dx of a position of ink landing relative to the position of ink ejection in the x direction is expressed by the expression (1)dx=(G/Vd)×Vs  (1)
Meanwhile, the technique put forth in the patent document 1 is configured to include an encoder for detecting a displacement of a carriage, a first counter for obtaining first position information by counting output pulses of the encoder and a second counter for obtaining second position information by counting output pulses of the encoder or counting pulses as the result of dividing the first position information as a configuration for detecting a position of a recording head.
Therefore, the inkjet recording apparatus according to the patent document 1 is capable of obtaining an appropriate position correction amount even if the speed of the carriage is changed (i.e., an acceleration and deceleration), and generating a recording timing signal in a higher frequency than an encoder frequency by generating a multiplied signal of the encoder output.
In an inkjet recording apparatus configured to array a plurality of short recording heads for jetting a single color ink, however, requires equipment of a number of recording heads with respectively different jet characteristics, and therefore a recording of an image requires a consideration of a difference in jet characteristic of each recording head and that of a change in recording density due to a mounting error of a plurality of short recording heads.
FIG. 2 exemplifies a configuration of a line head arraying a plurality of short recording heads for jetting a single color ink.
FIG. 2 shows a positional relationship between a recording medium 2 and a single color recording unit (i.e., a line head) arraying six short recording heads 11-1 through 11-6 (which are enclosed the by double-chain lines in the drawing) in the nozzle array direction (i.e., the x direction) staggeringly by separating them by the distance of dh in the conveying direction (i.e., the y direction).
In the inkjet recording apparatus comprising such a recording unit 12 for each color ink, if it is configured to array a plurality of recording units 12 for each ink color by separating in the conveying direction of the recording medium 2, an image is recorded at a recording timing at which a plurality of nozzles in each recording unit 12 is opposite a prescribed position of the recording medium 2.
FIG. 3A shows each conveying velocity of a recording medium 2 at a full line type image recording apparatus conveying the recording medium 2 in three kinds of relative move velocity areas (i.e., an acceleration area, a fixed velocity area and a deceleration area) relative to a recording unit 12 fixedly arraying the present recording unit 12 as shown in FIG. 2. Note that FIG. 3A shows the case of starting a record in the middle of the acceleration area and continuing it to the fixed velocity area, followed by ending the recording in the middle of the deceleration area.
Meanwhile, FIG. 3B illustrates a shift between the landing position on the recording medium 2 and the jetting position of a nozzle array at the time of the ink flying from the present nozzle array toward the recording medium 2 during the movement of the recording medium 2 at each of the above described relative speeds relative to the nozzle array of the recording unit 12.
FIG. 3B shows the fact that a shift amount between the jet position of the nozzle array and the landing position onto the recording medium 2 is changed in accordance with the relative move velocity (i.e., the velocity of the recording medium 2).
While the above described expression (1) has been described by exemplifying a serial scan type image recording apparatus in which the nozzle array moves, a replacement of the velocity Vs of the expression (1) with the conveying velocity of the recording medium makes it possible to calculate, by using the expression (1), a shift amount dy of the ink landing position in the relative move direction (i.e., the y direction) for a full line type image recording apparatus arraying fixedly the recording unit 12 as shown in FIG. 2.
As an example, calculating the dy by using the expression (1) in the following condition of:
relative move velocity Vs: 1 m/sec,
flying velocity of an ink droplet Vd: 10 m/sec, and
the distance between nozzle array and recording medium G: 2 mm, then
the shift amount dy in the relative move direction (i.e., the y direction): 200 micrometers.
The shift amount in the relative move direction calculated as described above is different among each of the above described relative move velocity areas (i.e., an acceleration area, a fixed velocity area and a deceleration area). If recorded by the above described full line type image recording apparatus in the acceleration area for example, the shift amount in the relative move direction is different with the relative move velocity of individual arraying position of the nozzle array, because the nozzle array of the short recording head for jetting a single color ink is arrayed separately in the front and rear (i.e., the upstream and downstream sides) of the conveying direction of the recording medium 2.
Meanwhile, when fixedly arraying the recording unit 12 as shown in FIG. 2 in the above described full line type image recording apparatus, a high precision adjustment mechanism is required to make the angle of a jet direction from each nozzle of the short recording heads 11-1 through 11-6 for example of the recording unit 12 entirely uniform and array, without allowing an error, the respective nozzle head parts separately arrayed in the front and rear of the conveying direction of the recording medium 2 so as not to overlap with one another when viewed in the y direction for example.
Also, the short recording heads 11 constituting such a recording unit 12 employs such as a method of using a piezoelectric element (PZT) that is an electro-mechanical transducer and of using a heat generated by a thermal resistor such as a thermal inkjet as the method for applying pressure to the ink in an ink fluid chamber. The nozzle array of the short recording head 11 is extremely finely formed, that is, the nozzle density of 150 to 600 dpi and the number of nozzles of several hundreds. This makes it extremely difficult to produce all the nozzles of the short recording head 11 uniformly in a nozzle plate. In a short recording head 11 using a PZT for example, a variation of jet velocity of each nozzle of each of short recording heads 11 may be generated by the material and/or process accuracy of the PZT.
There is a risk of the shift amount dy in the neighborhood of 10% in the relative move direction being generated by a variation of each nozzle array due to various causes. Because of this, if the shift amount in the relative move direction (i.e., the y direction) for example is 200 micrometers, the variation for each nozzle array is 20 micrometers. If such a full line type image recording apparatus records at the recording resolution of 600 dpi, there is a risk of an ink landing position shifting approximately by a half of the recording dot interval due to a variation of each nozzle array since the recording dot interval is approximately 42 micrometers.
Such a shift of an ink landing position of each nozzle array constitutes a cause for a non-uniform color due to a misalignment of overlapped colors especially for a full line type image recording apparatus arraying a plurality of the recording units 12 for each color of ink.